So, there's a few benefits to using Hemorrhage.
First off, it boosts the damage of your bleed effect - specifically, of Rupture and Garrote. Mainly Rupture is the important one here, because it's not an opener, and because, in fact, it's the highest damage/energy finisher you have available. Especially because, thanks to Sanguinary Vein, you'll always want to have a bleed effect up on your target in order to do more damage with all of your other abilities.
Thing is, the debuff from Hemo doesn't just increase the damage from your bleeds. It increases the damage of bleed effects applied by everyone else in the raid. That means Marksmanship Hunters, Warriors, other Rogues, and especially Feral Druids all will do a lot more damage in groups with you if you keep the Hemo debuff up. Hemorrhage is the Subtlety Rogues contribution to 'raid utility'.
Lastly, and most importantly for solo play, 5 mans, and leveling, is that with the Glyph of Hemorrhage, you can actually use Hemo to apply the bleed necessary to keep Sanguinary Vein active without worrying about using Rupture. This means that by using Hemo as your first unstealthed attack, the damage of all of your other abilities, including auto-attacks and poisons, goes up by 10% for 24 seconds - enough time to kill a short lived mob, and enough time to get a Rupture up on a boss that will live long enough for it to tick out.